Results from animal models and human studies suggest that drugs of abuse produce permanent structural and functional changes in the brain. We are analyzing the possible molecules and cells that might participate in temporal and permanent drug-induced alterations in rodent brains. Toward this goal, we are studying the spatio-temporal changes at the cellular level of specific neuronal molecules in brains of rats exposed to methamphetamine. Expression of the immediate early c-fos gene is commonly utilized as a high resolution histological marker of trans-synaptic neuronal stimulation. Previous studies have shown that acute methamphetamine administration induces expression of c-Fos immunoreactivity in the striatum. We used a combination of in situ hybridization and immunohistochemical methods to determine whether this sub-population of methamphetamine-responsive neurons contains either dynorphin (DYN) or enkephalin (ENK). These two neuropeptides are known to be present in GABAergic neurons in the striatum. Our preliminary analysis indicated that methamphetamine-induced c-Fos immunoreactivity was mainly confined to neurons that express pro-dynorphin. In contrast, a small number of c-Fos immunoreactive neurons express enkephalin. Our results thus provide anatomical evidence indicating that within the striatum, GABAergic neurons that contain DYN are responsive to acute methamphetamine (4 mg/kg) exposure. This observation is consistent with many pharmacological studies demonstrating the participation of striatal dynorphin system in the acute effects of amphetamine and methamphetamine. However, our anatomical findings do not support previous published anatomical results showing that amphetamine-induced c-Fos immunoreactivity was equally present in neurons expressing dynorphin and enkephalin.Nurr1 and Nur77 are orphan nuclear receptor transcription factors expressed in the adult mesolimbic dopamine system, this system is involved in reward mechanisms. In a time course study, we determined that a single IP injection of methamphetamine (4 mg/kg) induced up-regulation of Nur77 and Nurr1 mRNA. High levels of mRNA for both transcription factors were detected 2 hours after injection, while Nur77 mRNA expression was highly increased in the striatum and cortex, Nurr1 expression was mainly up-regulated in cortex. Expression for Nur77 and Nurr1 return to basal levels 6 hours after drug administration. Our preliminary results suggest that up-regulation of Nur77 mRNA take place in striatal DYN containing neurons, underscoring the importance of these neurons in the acute effects of methamphetamine. Outgoing studies are aimed to determine the phenotypic characteristics of neurons displaying methamphetamine-induced increases in Nur77 and Nurr1 transcripts in cortical areas.